Mathematical criterion for unsteady boundary-layer separation

Wang and Shen (1978) have observed that there is a certain coefficient which arises in the semisimilar formulation of the unsteady boundary layer problem and which apparently vanishes at the unsteady separation point. The present paper points out that there are additional solutions available which indicate a link between the vanishing of this coefficient and the separation point in some flows. It is also pointed out, however, that there are flows in which the coefficient vanishes but in which unsteady separation is neither expected nor indicated. The problem considered is that of investigating incompressible, two-dimensional, unsteady boundary-layer flows using the method of semisimilar solutions.